This invention relates to a method for reducing or eliminating the formation of sulfur dioxide during refiner bleaching with sodium hydrosulfite.
One of the first steps in making paper is the pulping of wood. The wood pulp created during the refining process is frequently not bright enough for use in paper formation without some type of treatment to brighten the pulp. Refiner bleaching is a widely accepted way of bleaching wood pulp. (See, for example, U.S. Pat. No. 5,129,987 to Joachimedes et al as one example of a paper and pulp refiner bleaching process. This process uses sodium hydrosulfite in the presence of a strong alkali such as NaOH.) During the process of refiner bleaching, one or more chemicals are added to the refiner. One such chemical is sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4, also called sodium dithionite. Due to problems associated with the decomposition of sodium hydrosulfite and the formation of sulfur dioxide during the extreme conditions of temperature and pressure present in a refiner, refiner bleaching has not enjoyed widespread popularity.
Sulfur dioxide is a corrosive and toxic chemical that combines with water to form sulfurous acid, another corrosive chemical. Pulp mills are especially sensitive to the formation of sulfur dioxide and the associated formation of sulfurous acid since most mills recover steam formed during the refining operation in order to reduce energy costs. Recovery and/or neutralization systems that render the acids harmless are very expensive. Also, at some point in the heat recovery system there will likely be some types of steels that are susceptible to attack by sulfur dioxide and sulfurous acid.
The continued requirement for increasingly higher brightness in paper has forced many manufacturers to look at alternatives to sodium hydrosulfite bleaching even though sodium hydrosulfite gives quite satisfactory results when used as a bleaching agent. Conventional tower bleaching with sodium hydrosulfite yields a brightness increase of about 6-10% based on International Standards Organization scale units (ISO) or as measured by other comparable scales such as Standard E.1 (including Appendices I and II) from the Physical and Chemical Standards Committee, Technical Section, Canadian Pulp and Paper Association, incorporated by reference in its entirety herein. An additional two points can be gained by splitting the hydrosulfite addition into two sequential dosages. Yet another two points of brightness can be achieved by performing the first stage of brightening in a mechanical pulping device such as a refiner or grinder, for example, of the type described in U.S. Pat. No. 5,129,987 to Joachimedes et al, (and its counterpart European Patent Application No. 0 333 398) incorporated by reference in its entirety herein. It would be advantageous, therefore, to continue the use of refiner bleaching with hydrosulfite if some way could be found to reduce or eliminate the formation of sulfur dioxide.
Thus, it is an object of this invention to provide a method for using sodium hydrosulfite in refiner bleaching processes without the formation of sulfur dioxide or with a reduction in the amount of sulfur dioxide formed. It is a further object of this invention to make the use of refiner bleaching with sodium hydrosulfite a more environmentally acceptable method of bleaching pulp. It is yet another object of the invention to enable the paper industry to obtain pulp having good brightness qualities. These and other objects of the invention will be apparent from the following description.